Asia Report: NTT Transmits at 3 Tb/s Using Hybrid TDM/WDM System
Combining a broadband supercontinuum WDM source with optical TDM technology, NTT researchers achieved a data rate of 3 Tb/s; Fujitsu reached 1.1 Tb/s in an all-WDM test.
By: Paul Mortensen
In a test designed to change the way most industry players think about high-speed data transmission, researchers at Nippon Telegraph and Telephone (NTT; Yokosuka, Japan) have transmitted data at 3 Tb/s (160 Gb/s over 19 channels) across 40 km of dispersion-shifted fiber. The record ultrahigh-bit-rate transmission demonstration combined optical time-division multiplexing (TDM) and wavelength division multiplexing (WDM) using two "supercontinuum" broadband WDM sources. According to NTT, terabit-rate optical transmission technology will be needed to meet the rapid growth in Internet traffic; their next target is 5 Tb/s.
Hybrid networks
Many researchers doubt that a substantial increase in transmission capacity can be achieved using conventional WDM, unless it is used in conjunction with optical TDM. Although the hybrid approach has been reported at a research level (see Multiwavelength Diode Laser Enables High-Speed Network), this is the first time that a commercial player in the communications industry has performed this type of demonstration. Hybrid networks rely on multiple semiconductor lasers whose oscillating wavelengths must be precisely selected and controlled, however, creating significant issues of reliability, stability, and the cost of the light sources.
NTT's solution is the supercontinuum (SC) pulse generator, which launches short light pulses into a nonlinear optical medium to produce a continuous optical spectrum that maintains its short-pulse nature. The supercontinuum light source can generate multiple-wavelength optical pulses with durations of less than 1 ps at arbitrary wavelengths, with a spectral stability about ten times that of a conventional semiconductor laser.
NTT believes that the source will play a major role in realizing low-cost, reliable large-capacity transmission based on hybrid technology. Incorporated in a network combining WDM with TDM, the 200-nm bandwidth of the source gives it a potential transmission capacity of 5 Tb/s.
Proof of concept
In their most recent test, NTT researchers transmitted a 3 Tb/s signal across a network comprising 19 WDM channels of 160 Gb/s optical TDM signals. The wavelengths of the 19 channels ranged from 1540 nm to 1609 nm, with a channel spacing of 480 GHz. The 3 Tb/s signal generator consists of two OTDM/WDM generators (see figure at top): one for the short wavelength region (1540 nm to1566 nm) and one for the long wavelength region (1570 nm to 1609 nm). Each generator incorporated a 10 GHz mode-locked erbium-doped fiber laser, an optical modulator, and an optical amplifier; to stabilize the SC output spectrum, all components of the generator were polarization-maintaining.
The generated 10 Gb/s SC signal pulses were then time-division multiplexed 16 times and spectrally sliced by array-waveguide generating filters, then recombined to form a 3 Tb/s WDM signal. This signal was then amplified by a 70-nm bandwidth tellurite-based erbium-doped fiber amplifier (EDFA) which provided 13 dB of flat gain across all 19 channels.
The total of 20.2 dB of optical power launched into the dispersion shifted fiber (DSF) transmission line during the experiment. The zero-dispersion wavelength of the 40 km fiber was 1535 nm, so all WDM channels were in the anomalous dispersion region; however, the team observed almost no degradation in the optical spectrum after transmission. At the receiver, each channel was split by an optical filter, optically pre-amplified, and then demultiplexed into a 10 Gb/s signal by an all-optical time domain demultiplexer.
NTT developed three proprietary technologies for the experiment: the supercontinuum light source, a wideband optical amplifier, and an optically-driven ultra-high-precision pulse separation technique. As for the future tests, the researchers expect to achieve longer transmission distance by using tellurite-based EDFAs as in-line repeaters, since these amplifiers provide flat gain over the range from 1540 to 1609 nm.
Meanwhile, at the competition…
Not all companies advocate NTT's hybrid approach, however. Using strictly WDM technology, Fujitsu Laboratories Ltd. (Atsugi, Japan) has achieved a transmission rate of 1.1 Tb/s. In their test, Fujitsu researchers used a total of 55 diode lasers operating in the 1.55 µm low-loss transmission window, with a channel spacing of 0.6 nm. The group chose newly-developed wideband EDFAs as post-amplifiers, and used in-line repeaters and a common pre-amplifier to increase signal strength simultaneously across all channels.
The transmission link consisted of 150 km of 1.3-µm zero-dispersion singlemode fiber with an amplifier spacing of 50 km. Lithium niobate Mach-Zehnder type external modulators produced a data rate of 20 Gb/s for each signal, for a total capacity of 1.1 Tb/s.
About the author…
Paul Mortensen is a freelance science writer based in Australia. He can be reached at paulmn@ozemail.com.au.